1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier or a printer, using an electrophotographic process.
2. Description of the Related Art
Conventionally, there have been known color image forming apparatuses having image forming means for forming images of four colors consisting of yellow (Y), magenta (M), cyan (C), and black (Bk). Those color image forming apparatuses perform image formation by transferring toner images, which are born on image bearing members for the respective colors, to an intermediate transfer unit, and transferring the toner images on the intermediate transfer unit to a sheet.
Such color image forming apparatuses have the problem that color shift occurs when image forming positions of the four colors become misaligned, and the color shift presents itself in an image, resulting in degradation of image quality. The color shift is caused by shaft deflection, nonuniform rotation, nonuniform speed, etc. of the image bearing members which are rotating.
As measures concerning the color shift, there have been proposed, for example, a method to prevent the color shift by individually controlling rotation phases of the image bearing members for the respective colors. For example, a group of color image bearing members on which color images are formed and a black image bearing member on which a black image is formed are driven by motors which are different rotatably driving means, and the motor that drives the black image bearing member drives an intermediate transfer unit as well. There has been proposed an adjustment method that, in the arrangement described above, the rotation phase of the motor that drives the black image bearing member and the intermediate transfer unit is used as the reference, and the rotation phase of the motor that drives the group of color image bearing members is matched with the rotation phase of the motor that drives the black image bearing member and the intermediate transfer unit (see Japanese Laid-Open Patent Publication (Kokai) No. 2008-197146, for example).
According to the technique disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2008-197146, control has to be performed so as to match the rotation phase of the motor that drives the black image bearing member and the rotation phase of the motor that drives the group of color image bearing members with each other. For this reason, control is performed to activate the motors step by step so as to minimize the likelihood of phase shift during the activation. More specifically, to reduce convergence time, the phase control at the activation of the motors is started with reference to the motor that drives the black image bearing member at a time point each motor reaches a predetermined speed, and as soon as the phase control is completed within a predetermined time period, image formation is started.
However, there is the problem that if the phase control for the color motor is performed in a monochrome mode in which images are formed in only black color, this will end up increasing first print output time. Also, there is the problem that if the rotational speed of the color motor is higher than that of the motor that drives the black image bearing member, the intermediate transfer unit will slack due to rotation of the color motor following the movement of the intermediate transfer belt because the motor that drives the black image bearing member drives the intermediate transfer unit as well.